Chronic beryllium disease (CBD) is a granulomatous lung disorder caused by beryllium exposure in the workplace and is characterized by the accumulation of large numbers of beryllium-specific CD4* T cells in the lung. Due to unique chemical and physical properties, beryllium continues to be utilized in hightechnology industries. Thus, CBD remains an important public health concern with more than 1,000,000 workers having been exposed to beryllium and at risk for disease development. Beryllium sensitization is detected by the ability of blood CD4* T cells to proliferate in the presence of beryllium salts in culture. A subset of beryllium-sensitized (BeS) individuals progress to CBD at a rate of 6-8% per year, and the natural history of CBD is characterized by the development of lung fibrosis. What factors are involved in the progression from beryllium sensitization to disease and how various T cell subsets play a role in this progression remain important unanswered questions and form the basis of this application. We have recently shown that the frequency of beryllium-responsive CD4^ T cells in blood of CBD patients is significantly greater than that found in BeS subjects and directly correlates with markers of lung inflammation, suggesting that the number of beryllium-specific T cells in the circulating pool reflects the severity of the CD4* T cell alveolitis. In addition, our preliminary data suggest that the quantity of naturally-occurring, Foxp3-expressing T regulatory (Treg) cells in the lung of CBD patients is diminished compared to the lung of BeS subjects. We hypothesize that progression from beryllium sensitization to CBD is due to the presence of deficient and/or dysfunctional naturally-occurring Treg cells in lung. As a direct consequence, an increased number of pathogenic beryllium-responsive CD4^ T cells accumulate in blood and lung, associated with an exaggerated T cell dependent immune response. The first specific aim will analyze the role of naturally-occurring, Foxp3-expressing Treg cells in disease progression. Specific aim 2 will determine whether the frequency of circulating beryllium-responsive CD4'' T cells serves as a biomarker of disease progression and severity while the final aim will evaluate the effects of the anti-TNF-a mAb, infliximab, on the frequency and function of beryllium-responsive CD4* T cells and naturally-occurring regulatory CD4* T cells in blood and BAL of CBD patients. Thus, the development of intermediate biomarkers capable of detecting disease progression in high-risk individuals and the ability of monitor these biomarkers after treatment initiation would represent a tremendous advance in the field of granulomatous lung disease.